There are many ways of producing perforate solid materials useful for separation devices and the like, but such procedures generally cannot produce very thin perforate membranes having very narrow pores which are desired in many filtering and similar applications in order, for example, to reduce back pressures, to increase concentration gradients, and to enable smaller particles to be separated.
It is known that porous oxide films can be grown on certain metals, most notably aluminum and anodizable aluminum alloys, by anodizing such metals in electrolytes containing strong acids, such as sulfuric acid and phosphoric acid and films of this type have been suggested for use in filter devices (see U.S. Pat. No. 3,850,762 issued on Nov. 26, 1974 to A. W. Smith). Anodic films can be made very thin (e.g. in the order of nanometers) and can be grown to any desired larger thicknesses by continuing the anodization process for a suitable length of time. However, it has not generally been convenient to use such films for separation devices and other similar purposes for two basic reasons. First of all, anodic films adhere tenaciously to the underlying metal on which they are formed and cannot easily be removed without resorting to dissolving away the metal substrate (a slow and expensive procedure). Secondly, anodic films produced by porous anodization have imperforate barrier layers at the bases of the pores, immediately adjacent to the metal substrate, so this layer has to be removed if the film is to be used in a device which requires complete penetration of the film, but its removal is very difficult without damaging the remaining porous structure.
We have previously found (see, for example, our European Patent Application Serial No. 0 178 831 published on Apr. 23, 1986, the disclosure of which is incorporated herein by reference) that the adhesion of an anodic film to an underlying metal substrate can be considerably reduced by a pore branching procedure carried out during the porous anodization step, and that the anodic film (which detaches spontaneously or after a period of soaking) thus produced has pores which extend completely from one surface of the film to the other. It is suggested in the published application that the porous anodic films are self-supporting when thick enough (e.g. 50 microns or thicker) but that thin films (0.1 microns to 50 microns) may need to be supported on a porous substrate. It is not easy, however, to attach such free standing films to a suitable support nor to detach such films from the underlying metal in a reliable way when large areas are involved. Films of less than about 25 microns in thickness are particularly difficult to handle in a manufacturing environment. Consequently, porous anodic films have not found widespread use as porous membranes for filters and other devices.